Alongside developing websites, I've been teaching web design to undergraduates for a number of years - 12 years to date! Unlike some subjects, web changes rapidly and certainly makes teaching the subject challenging - and fun too of course!
During a lecture on the topic of Responsive Web Design, the students and I discussed the impact that responsive web has on development time of the humble web designer. In the past, it may have been acceptable to build a fixed width layout, e.g. one that stays the same across all browsers and devices or offer a mobile only site as an optional extra. However, today, the current standard demands a website look equally great across all browsers and devices. This is a wonderful improvement on user experience, but what impact does this had on website development time?
Enter the CSS Framework. A CSS Framework, such as Bootstrap, provides a number of pre-written html elements and css classes to help speed up development time. *Gasp* say the students "But isn't that sort of cheating!". Well, that depends on a number of factors. For example, as I said to my students, if you're being graded on hand-coding a site 'from scratch', then technically, yes this would be considered 'cheating'. However, what are the merits of reinventing the wheel? So you now know 'how' to hand-code a site from scratch, so what are you gaining from carrying out the same repetitive tasks over and over again for every project? We discussed the the principle of D.R.Y - don't repeat yourself - in relation to developing a responsive website using a grid system and the students started to see the benefits - cut out the boring admin tasks such as reseting browser defaults, and floating columns etc. and spend more time being creative and working towards meeting a client brief.
Bootstrap is one of the market leaders of css frameworks, and a skill often requested by many job adverts for front-end web designers and developers globally. It is easy to get started, and wireframing a mobile-first responsive website is far quicker than writing a framework from scratch. My students enjoyed learning about css frameworks, and were particularly interested to note that this solution doesn't 'create a site for you' - it simply makes life a little easier.
Whether you favour a css framework approach or not, it really comes down to two things - personal preference - if you'd rather hand-code go for it!, and what is your clients budget - how much time can you afford to spend developing the site based on the clients budget.